Nonresponsiveness of the rat hypothalamo-pituitary-adrenocortical axis to parturition-related events: Inhibitory action of endogenous opioids

During the last 2 days of pregnancy in rats, basal corticosterone secretion is enhanced, although the response of the hypothalamo-pituitary-adrenocort ical (HPA) axis to emotional and physical stressors is blunted, independent of the action of endogenous opioids. In this study, alterations in the rea ctivity of the HPA axis, which may accompany parturition-related stimuli, a nd the involvement of endogenous opioids were examined in chronically cathe terized rats. In vehicle-treated controls (n = 9), ACTH and corticosterone secretion decr eased in preparation for birth (P < 0.01) and further declined immediately after delivery of the second pup (P < 0.01), remaining low for 150 min. In contrast, in animals injected with the opiate antagonist naloxone (5 mg ml( -1) kg(-1), iv, n = 6) after delivery of the second pup, ACTH and corticost erone release were enhanced within 20 min (ACTH, 5.0-fold; corticosterone, 2.3-fold; P < 0.01 vs. controls) and returned to control levels after 90 mi n. In confirmation of previous reports, oxytocin secretion into blood was e levated in control rats after the onset of parturition (P < 0.01) and was f urther enhanced in the naloxone group (1.4-fold, P < 0.01 vs. control). Pla sma lactate concentration was increased, 30 min aff er the onset of deliver y (1.9-fold, P < 0.01), independent of the treatment. The data indicate tha t parturition-related events do not trigger HPA axis hormone release becaus e of an effective inhibition by endogenous opioids. This nonresponsiveness of the HPA axis is likely to protect the pups' well-being during birth.